The esophagus of the opossum in vitro and in vivo to discover the neuromuscular mechanisms underlying esophageal motility in swallowing. Other gastrointestinal viscera are examined to seek resemblances to or differences from the esophagus, as appropriate. Other species are examined as well; these include cat and monkey. BIBLIOGRAPHIC REFERENCES: De Carle, D., and Christensen, J.: Histamine Receptors in Opossum Esophageal Smooth Muscle. Physiologist, 18:186, 1975. Christensen, J., and de Carle, D.J.: Letter to the Editor. Comparative Anatomy of the Esophagus. Gastroenterology, 67: 407-408, 1974.